The Flash

"Elseworlds, Part 1" kicks off a highly anticipated new three-night crossover event that opens when Barry Allen and Oliver Queen (Grant Gustin, guest star Stephen Amell) awaken to realize something has caused them to switch bodies with each other. They’re desperate to understand what upset the timeline to cause this shift, but when the members of Team Flash don’t believe their story, the two heroes realize they need the help of Supergirl (guest star Melissa Benoist). The saga continues Monday on "Arrow," then concludes Tuesday on "Supergirl."